It is well known in the art to produce a relatively large diameter hole between a first location and a second location in a mine or other underground works by an operation commonly referred to as raise drilling. A raise drilling operation begins by drilling a small diameter pilot hole through the earth from a first location to an opening at the second location using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill string and a large diameter raise head or raise bit is attached to the drill string. The raise head is rotated and drawn along the pilot hole thereby enlarging the pilot hole to the desired size.
Many strict requirements are imposed upon a raise head that is used in boring a large diameter raise hole. The raise head must be a balanced, high-performance apparatus that is rugged and will perform for a long period of time. Raise boring heads of this type use replaceable rolling cutters located and space so that upon rotation of the raise head every portion of the hole being drilled will be acted upon by one or more of the cutters in order to disintegrate the formations and form the desired raise hole. This insures that almost the entire wear in drilling takes place on the cutters rather than on the main raise head body. The cutters are readily replaceable thereby allowing the life of the raise head to be extended by replacing the individual cutters.
Traditionally, raise boring heads have been manufactured by welding together a series of plates, gussets, stiffeners, and rolls. Each individual cutter saddle location was separately machined. This involved substantial problems because of the time for set-up and machining. The body of the prior art raise boring heads included a network of horizontal plates arranged around the central drive stem. This did not permit all of the falling debris to escape the confines of the body portion of the raise boring head. The build-up of debris impeded performance of the raise head and reduced the life of the head.